EXPLAINER-Why it’s so hard to diagnose Zika

CHICAGO, Oct 17 (Reuters) - When a Zika epidemic was at its height in the Americas two years ago, diagnostics makers began working feverishly to create diagnostic tests for a virus that few in the U.S. had heard of.

Those efforts have now largely stalled, as public concern has waned, health experts say, and the development of inexpensive tests that can quickly detect Zika infections and distinguish them from similar mosquito-borne diseases remains elusive.

A lack of testing capacity has hampered efforts to track Zika in Angola, where a largely unreported cluster of microcephaly cases has been linked to the virus, and left mothers vulnerable to an illness that can cause severe birth defects in developing fetuses.

WHY IS IT SO HARD TO TEST FOR ZIKA?

The best and easiest way to detect a Zika infection is to test for the virus while a person is still symptomatic. This is done through molecular tests, such as polymerase chain reaction or PCR, which look for genetic material from the virus. Such tests have a very short window and must be done during the seven to 14 days when the virus is active.

Given that Zika typically causes only mild illness, or no symptoms at all, many people who have been infected miss this critical window.

After the acute infection passes, doctors use so-called serology tests, which sample blood or plasma for antibodies produced in response to the virus.

The drawback with these tests is that Zika is very similar to dengue and other viruses in the same family that are also endemic to tropical regions. Antibodies made in response to these infections can cross-react, producing false positive results.

Because of the potential for cross-reactivity, positive serology results are tested again with a more sensitive, confirmatory test called a plaque reduction neutralization test or PRNT. Such tests are labor-intensive and only performed by select public health laboratories.

WHY IS THE EFFORT IMPORTANT?

Dr. Eve Lackritz, who leads the Zika Task Force at the World Health Organization, says promoting better diagnostics is a top priority to help track the spread of the virus and develop vaccines. WHO convened a meeting on diagnostics in Geneva last week with global public health officials, test makers and funding agencies.

“This is not the time to step away,” Lackritz said in an interview. “We will see growing populations of susceptible people who can get infected.” Better tests are needed to check women, especially those who are asymptomatic, during pregnancy, she said. That would include tests to distinguish Zika from other, similar viruses. Another need is for quick, inexpensive tests that could be used easily in a doctor’s office in developing countries.

HOW FAR HAVE WE COME?

Nearly two dozen manufacturers responded to WHO’s call in 2016 to develop Zika tests. By 2017, the U.S. Food and Drug Administration had granted special emergency use approval to 19 diagnostic tests.

The designation allowed companies such as Quest Diagnostics , Chembio Diagnostics, Luminex Corp, Siemens Healthineers, Abbott Molecular, and InBios International to sell their tests without a full review of their effectiveness. “They got to a certain point, but didn’t get actual regulatory approval,” said Maurine Murtagh, a former director of diagnostic services for the Clinton Health Access Initiative. The absence of a large outbreak and waning support from funding agencies has removed a lot of the incentive for companies to press on, she said.

Chembio, maker of the only rapid Zika test to receive FDA emergency use authorization, is moving forward with plans to seek full U.S. regulatory approval. Health authorities in Europe and Brazil have approved a different version of the company’s test.

Chief Executive John Sperzel said that while governments will fund the development and purchase of vaccines in a public health emergency, diagnostics companies rarely see such support. Chembio was one of a handful of companies to get a U.S. grant to work on Zika.

That’s an issue Lackritz is trying to address. “People need to understand there needs to be market forces to get companies to discover diagnostics,” she said. (Reporting by Julie Steenhuysen; Editing by Michele Gershberg and Sue Horton)